Mars was only 18 when he signed a $100,000 recording deal in 2004 with Motown. The budding singer-songwriter was elated to earn a contract with the fabled record label, once home to such legends as Marvin Gaye, Diana Ross and Michael Jackson, Mars’ biggest musical inspiration.

Eight months later, after a series of Los Angeles recording sessions that yielded repeated frustration for the young Hawaiian native, Motown dropped him. For Mars (real name: Peter Gene Hernandez Jr.), it was a professional disaster that put him on the road to international stardom.

“It was probably the best thing that ever happened to me, as far as being a reality check and (learning) what it really takes to break into this (business),” said Mars, 25, who plays a nearly sold-out show here Saturday at the San Diego County Fair’s 9,000-capacity Grandstand Stage. Janelle Monae opens.

“I was so young at the time that, at 18, I probably thought being signed (to Motown) was it for me," Mars continued, speaking from a recent Seattle concert stop. "But you don’t know what the hell you’re doing at 18. And when I got dropped, that’s when I thought: ‘I’ll do everything on my own; I’ll write my own music and produce it.’

"I was very frustrated (at Motown) because I was in (recording) studios with producers, trying to explain to them how I wanted these songs to sound. And how can you explain something you haven’t done yet?”

Mars hasn’t done it all, yet, not even close.

But the rising young star has become one of pop music’s most in-demand collaborators, thanks to his key singing, songwriting and arranging contributions to such hit songs as Cee Lo Green’s Grammy-winning “F--- You” (later retitled “Forget You”), Travis McCoy’s “Billionaire,” B.o.B’s “Nothin’ On You” and Flo Rida’s “Right Round.”

“I’d like to collaborate with Alicia Keys and Kings of Leon,” Mars said. “I recently did a song with Eminem and would love to do another. … My goal is to find a new artist; I’d love to produce a whole album that I’m not singing on.”

Like very few of his contemporaries, Mars is equally adept creating hip-hop, pop, neo-soul, reggae or rock (both contemporary and retro).

His versatility is due, in part, to his having launched his musical career soon after he stopped wearing diapers. By age 3, Mars was singing “Hound Dog” and shaking his hips, mini-Elvis-style, as a member of his parents’ Honolulu lounge band, the Love Notes. He was not yet 4 when he briefly appeared as the world's youngest Elvis impersonator on the Nicolas Cage/Sarah Jessica Parker film comedy, "Honeymoon in Vegas." (Click here to watch his precocious screen debut.)